It is well known to use a spark plug to ignite an air and fuel mixture in a combustion chamber in a spark ignition engine.
The performance of the spark plug can directly affect engine performance; if the spark plug does not properly ignite the air and fuel mixture in the combustion chamber, there will be fuel at least partially unburned, leading to a loss of power, reduced fuel efficiency and increased hydrocarbon emissions.
Given the extreme conditions under which a typical spark plug operates, and given that a typical spark plug fires many times before replacement, deterioration is inevitable; this often manifests as deposits on the electrodes, and also manifests as erosion of the electrodes.
Various innovations have been proposed to reduce deterioration.
Many innovations have been focused towards the issue of electrode erosion: toughening the material from which the electrodes are manufactured or increasing the number of electrodes.
Another innovation is the surface discharge spark plug. In a surface discharge spark plug, the metallic outer shell of the spark plug presents an annular ground electrode disposed coaxially with a central positive electrode.
Specific innovations to the construction of the annular grounding electrode have also been proposed. It is known, for example, to provide open ended slots, backward spaced slots and/or holes defined by the annual grounding electrode. Each of these techniques incorporates a complicated re-design of a conventional spark plug that could result in significantly higher manufacturing costs.